php file manipulation

$line_counter = 0;
$desired_line = 29;
$fh = fopen('vacation-hotspots.txt','r') or die($php_errormsg);
while ((! feof($fh)) && ($line_counter <= $desired_line)) {
if ($s = fgets($fh,1048576)) {
$line_counter++;
}
}
fclose($fh) or die($php_errormsg);
print $s;
AND:
Modifying a File in Place Without a Temporary File:
$fh = fopen('message.txt','r+') or die($php_errormsg);
// figure out how many bytes to read
$bytes_to_read = filesize('message.txt');
// initialize variables that hold file positions
$next_read = $last_write = 0;
// keep going while there are still bytes to read
while ($next_read < $bytes_to_read) {
/* move to the position of the next read, read a line, and save
* the position of the next read */
fseek($fh,$next_read);
$s = fgets($fh,1048576) or die($php_errormsg);
$next_read = ftell($fh);
// convert <b>word</b> to *word*
$s = preg_replace([EMAIL="'@<b[^>]*>(.*?)</b>@i','*$1*',$s"]'@<b[^>]*>(.*?)</b>@i','*$1*',$s[/EMAIL]);
// convert <i>word</i> to /word/
$s = preg_replace([EMAIL="'@<i[^>]*>(.*?)</i>@i','/$1/',$s"]'@<i[^>]*>(.*?)</i>@i','/$1/',$s[/EMAIL]);
/* move to the position where the last write ended, write the
* converted line, and save the position for the next write */
fseek($fh,$last_write);
if (-1 == fwrite($fh,$s)) { die($php_errormsg); }
$last_write = ftell($fh);
}
// truncate the file length to what we've already written
ftruncate($fh,$last_write) or die($php_errormsg);
// close the file
fclose($fh) or die($php_errormsg);

both work well to read a specific line of the file and the other to replace a part of the file or all the file, could some one tell me a function to combine that i mean a function to replace part of a line or all the line of an opened file but just in the line that i want,
thanks a lot